forked from lijiext/lammps
163 lines
6.9 KiB
HTML
163 lines
6.9 KiB
HTML
<HTML>
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<HR>
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<H3>fix efield command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID efield ex ey ez keyword value ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>efield = style name of this fix command
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<LI>ex,ey,ez = E-field component values (electric field units)
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<LI>any of ex,ey,ez can be a variable (see below)
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<LI>zero or more keyword/value pairs may be appended to args
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<LI>keyword = <I><I>energy</I>
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<PRE> <I>energy</I> value = v_name
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v_name = variable with name that calculates the potential energy of each atom in the added E-field
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix kick external-field efield 1.0 0.0 0.0
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fix kick external-field efield 0.0 0.0 v_oscillate
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Add a force F = qE to each charged atom in the group due to an
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external electric field being applied to the system. If the system
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contains point-dipoles, also add a torque on the dipoles due to the
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external electric field.
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</P>
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<P>For charges, any of the 3 quantities defining the E-field components
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can be specified as an equal-style or atom-style
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<A HREF = "variable.html">variable</A>, namely <I>ex</I>, <I>ey</I>, <I>ez</I>. If the value is a
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variable, it should be specified as v_name, where name is the variable
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name. In this case, the variable will be evaluated each timestep, and
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its value used to determine the E-field component.
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</P>
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<P>For point-dipoles, equal-style variables can be used, but atom-style
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variables are not currently supported, since they imply a spatial
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gradient in the electric field which means additional terms with
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gradients of the field are required for the force and torque on
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dipoles.
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</P>
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<P>Equal-style variables can specify formulas with various mathematical
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functions, and include <A HREF = "thermo_style.html">thermo_style</A> command
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keywords for the simulation box parameters and timestep and elapsed
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time. Thus it is easy to specify a time-dependent E-field.
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</P>
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<P>Atom-style variables can specify the same formulas as equal-style
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variables but can also include per-atom values, such as atom
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coordinates. Thus it is easy to specify a spatially-dependent E-field
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with optional time-dependence as well.
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</P>
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<HR>
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<P>Adding a force or torque to atoms implies a change in their potential
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energy as they move or rotate due to the applied E-field.
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</P>
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<P>For dynamics via the "run" command, this energy can be optionally
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added to the system's potential energy for thermodynamic output (see
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below). For energy minimization via the "minimize" command, this
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energy must be added to the system's potential energy to formulate a
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self-consistent minimization problem (see below).
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</P>
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<P>The <I>energy</I> keyword is not allowed if the added field is a constant
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vector (ex,ey,ez), with all components defined as numeric constants
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and not as variables. This is because LAMMPS can compute the energy
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for each charged particle directly as E = -x dot qE = -q (x*ex + y*ey
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+ z*ez), so that -Grad(E) = F. Similarly for point-dipole particles
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the energy can be computed as E = -mu dot E = -(mux*ex + muy*ey +
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muz*ez).
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</P>
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<P>The <I>energy</I> keyword is optional if the added force is defined with
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one or more variables, and if you are performing dynamics via the
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<A HREF = "run.html">run</A> command. If the keyword is not used, LAMMPS will set
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the energy to 0.0, which is typically fine for dynamics.
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</P>
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<P>The <I>energy</I> keyword is required if the added force is defined with
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one or more variables, and you are performing energy minimization via
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the "minimize" command for charged particles. It is not required for
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point-dipoles, but a warning is issued since the minimizer in LAMMPS
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does not rotate dipoles, so you should not expect to be able to
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minimize the orientation of dipoles in an applied electric field.
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</P>
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<P>The <I>energy</I> keyword specifies the name of an atom-style
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<A HREF = "variable.html">variable</A> which is used to compute the energy of each
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atom as function of its position. Like variables used for <I>ex</I>, <I>ey</I>,
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<I>ez</I>, the energy variable is specified as v_name, where name is the
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variable name.
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</P>
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<P>Note that when the <I>energy</I> keyword is used during an energy
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minimization, you must insure that the formula defined for the
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atom-style <A HREF = "variable.html">variable</A> is consistent with the force
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variable formulas, i.e. that -Grad(E) = F. For example, if the force
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due to the electric field were a spring-like F = kx, then the energy
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formula should be E = -0.5kx^2. If you don't do this correctly, the
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minimization will not converge properly.
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</P>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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</P>
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<P>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>.
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</P>
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<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option is supported by this
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fix to add the potential "energy" inferred by the added force due to
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the electric field to the system's potential energy as part of
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<A HREF = "thermo_style.html">thermodynamic output</A>. This is a fictitious
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quantity but is needed so that the <A HREF = "minimize.html">minimize</A> command
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can include the forces added by this fix in a consistent manner.
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I.e. there is a decrease in potential energy when atoms move in the
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direction of the added force due to the electric field.
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</P>
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<P>This fix computes a global scalar and a global 3-vector of forces,
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which can be accessed by various <A HREF = "Section_howto.html#howto_15">output
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commands</A>. The scalar is the potential
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energy discussed above. The vector is the total force added to the
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group of atoms. The scalar and vector values calculated by this fix
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are "extensive".
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</P>
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<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
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the <A HREF = "run.html">run</A> command.
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</P>
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<P>The forces due to this fix are imposed during an energy minimization,
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invoked by the <A HREF = "minimize.html">minimize</A> command. You should not
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specify force components with a variable that has time-dependence for
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use with a minimizer, since the minimizer increments the timestep as
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the iteration count during the minimization.
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</P>
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<P>IMPORTANT NOTE: If you want the fictitious potential energy associated
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with the added forces to be included in the total potential energy of
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the system (the quantity being minimized), you MUST enable the
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<A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option for this fix.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>This fix is part of the MISC package. It is only enabled if LAMMPS
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was built with that package. See the <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "fix_addforce.html">fix addforce</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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